return empty list instead of null in MPD.getArtists(Genre)
return empty list instead of null in MPD.getArtists(Genre)

abarisain/dmix,philchand/mpdroid-2014,0359xiaodong/dmix,hurzl/dmix,joansmith/dmix,hurzl/dmix,joansmith/dmix,philchand/mpdroid-2014,philchand/mpdroid-2014,philchand/mpdroid-2014,jcnoir/dmix,0359xiaodong/dmix,abarisain/dmix,jcnoir/dmix